Bicycle Mechanics - No backpedaling on a Tiagra?

I recently bought my first semi-decent bike (Sirrus Sport), and I am a big fan for commuting. However, I am concerned about the rear derailleur. It sits really really close to the cartridge, to where I cannot backpedal - the chain stops altogether and goes slack until I start pedaling forward. Do I just need to adjust the B screw, or is there something else going on here?

Yeah, there's certainly something wrong if you can't backpedal without problems. The B-limit screw would probably be the first place I looked as well, though also may make sure that the rear wheel is seated correctly in the dropouts.

Since you don't specify, I assume you're unable to backpedal in any of your gears? It sounds like because the upper derailler pully -wheel is too close to the sprockets (rear gears). In that case, the B-limit screw is a good place to try. You could also shorten the chain if it's not already at the shortest possible length that allows you to use your most-total-gear-teeth combination... shorter chain will pull the derailler cage more away from the sprockets.

It's also possible that the freehub is gummed up. If the chain is off, are you able to spin the gears backwards?

I recently bought my first semi-decent bike (Sirrus Sport), and I am a big fan for commuting. However, I am concerned about the rear derailleur. It sits really really close to the cartridge, to where I cannot backpedal - the chain stops altogether and goes slack until I start pedaling forward. Do I just need to adjust the B screw, or is there something else going on here?

You have described normal function for a bicycle equipped with a triple crank and short chainstays. I wouldn't imagine it does this in all gears, just the ones where the chain angle is more severe. If the upper pulley does not make contact with the cassette when in the largest cog, then the B-tension is OK.

It's certainly possible you have a bad freehub body, but my money's on the triple/short chainstay as the problem.

Agree that the chain length is probably wrong. Or the deraileur is not hanging directly below the cog it is supposed to be on. LBS should be able to remedy in a few minutes.

I recently bought my first semi-decent bike (Sirrus Sport), and I am a big fan for commuting. However, I am concerned about the rear derailleur. It sits really really close to the cartridge, to where I cannot backpedal - the chain stops altogether and goes slack until I start pedaling forward. Do I just need to adjust the B screw, or is there something else going on here?

Is this a brand new from a bike shop bike? If so, take it back and give them a chance to fix it for you. Screwing around willy-nilly with rear derailleur adjustments is more likely to make it worse than it is to make it better.